1. Field of the Invention
The present invention relates to a thin film actuated mirror array in an optical projection system, and more particularly to a thin film actuated mirror array in an optical projection system effectively preventing point defects of pixels and enhancing the quality of a picture projected onto a screen.
In general, light modulators are divided into two groups according to their optics. One type is a direct light modulator such as a cathode ray tube (CRT) and the other type is a transmissive light modulator such as a liquid crystal display (LCD). The CRT produces superior quality pictures on a screen, but the weight, the volume and the manufacturing cost of the CRT increase according to the magnification of the screen. The LCD has a simple optical structure, so the weight and the volume of the LCD are less than those of the CRT. However, the LCD has a poor light efficiency of under 1 to 2% due to light polarization. Also, there are some problems in the liquid crystal materials of the LCD such as sluggish response and overheating.
Thus, a digital mirror device (DMD) and actuated mirror arrays (AMA) have been developed in order to solve these problems. At the present time, the DMD has a light efficiency of about 5% and the AMA has a light efficiency of above 10%. The AMA enhances the contrast of a picture on a screen, so the picture on the screen is more apparent and brighter. The AMA is not affected by and does not affect the polarization of light and therefore, the AMA is more efficient than the LCD or the DMD.
FIG. 1 shows a schematic diagram of an engine system of a conventional AMA which is disclosed in U.S. Pat. No. 5,126,836 (issued to Gregory Um). Referring to FIG. 1, a ray of incident light from light source 1 passes a first slit 3 and a first lens 5 and is divided into red, green, and blue lights according to the Red Green Blue (R G B) system of color representation. After the divided red, green, and blue lights are respectively reflected by a first mirror 7, a second mirror 9, and a third mirror 11, the reflected lights are respectively incident on AMA devices 13, 15 and 17 corresponding to the mirrors 7, 9 and 11. The AMA devices 13, 15 and 17 tilt the mirrors installed therein, so the incident light is reflected by the mirrors. In this case, the mirrors installed in the AMA devices 13, 15 and 17 are tilted according to the deformation of active layers formed under the mirrors. The light reflected by the AMA devices 13, 15 and 17 pass a second lens 19 and a second slit 21 and form a picture on a screen (not shown) by using projection lens 23.
In most cases, ZnO is used as the active layer. However, lead zirconate titanate (PZT:Pb(Zr,Ti)O.sub.3) has a better piezoelectric property than ZnO. PZT is a complete solid solution of lead zirconate (PbZrO.sub.3) and lead titanate (PbTiO.sub.3). A cubic structure PZT exists in a paraelectric phase at a high temperature. An orthorhombic structure PZT exists in an antiferroelectric phase, a rhombohedral structure PZT exists in a ferroelectric phase, and a tetragonal structure PZT exists in a ferromagnetic phase according to the composition ratio of Zr and Ti at a room temperature. A morphotropic phase boundary (MPB) of the tetragonal phase and the rhombohedral phase exists as a composition which includes Zr:Ti at a ratio of 1:1. PZT has a maximum dielectric property and a maximum piezoelectric property at the MPB. The MPB exists in a wide region in which the tetragonal phase and the rhombohedral phase coexist, but does not exist at a certain composition. Researchers do not agree about the composition of the phase coexistent region of PZT. Various theories such as thermodynamic stability, compositional fluctuation, and internal stress have been suggested as the reason for the phase coexistent region. Nowadays, a PZT thin film is manufactured by various processes such as spin coating method, organometallic chemical vapor deposition (OMCVD) method, and sputtering method.
The AMA is generally divided into a bulk type AMA and a thin film type AMA. The bulk type AMA is disclosed in U.S. Pat. No. 5,469,302 (issued to Dae-Young Lim). In the bulk type AMA, after a ceramic wafer which is composed of a multilayer ceramic inserted into metal electrodes therein is mounted on an active matrix having transistors, a mirror is mounted on the ceramic wafer by means of sawing the ceramic wafer. However, the bulk type AMA has disadvantages in that it demands a very accurate process and design, and the response of an active layer is slow. Therefore, the thin film AMA which is manufactured by using semiconductor technology has been developed.
The thin film AMA is disclosed at U.S. Pat. No. 5,815,304 (issued to CHI) entitled "THIN FILM ACTUATED MIRROR ARRAY IN AN OPTICAL PROJECTION SYSTEM AND METHOD FOR MANUFACTURING THE SAME".
FIG. 2 is a plan view for showing the thin film AMA, FIG. 3 is a perspective view for showing the thin film AMA in FIG. 2 and FIG. 4 is a cross-sectional view taken along line A.sub.1 -A.sub.2 of FIG. 3.
Referring to FIGS. 2 to 4, the thin film AMA has a substrate 31, an actuator 57 formed on the substrate 31, and a reflecting member 55 formed at a central portion of the actuator 57.
The substrate 31 including electrical wiring (not shown) has a connecting terminal 33 formed on the electrical wiring, a passivation layer 35 overlayed on the substrate 31 and on the connecting terminal 33, and an etching stop layer 37 overlayed on the passivation layer 35. The actuator 57 has a supporting layer 43, a bottom electrode 45, an active layer 47, a top electrode 49 and a via contact 53.
Referring to FIG. 3, the supporting layer 43 has a first portion attached beneath the bottom electrode 45 and a second portion exposed out of the bottom electrode 45. Bottoms of both lateral borders of the supporting layer 43 are partially attached to the etching stop layer 37. The attached portions of the supporting layer 43 are called anchors 43a, 43b which support the actuator 57. The lateral borders of the supporting layer 43 are parallelly prolonged from the attached portions. The central portion of the supporting layer 43 is integrally formed with the lateral borders between the lateral borders. The central portion of the supporting layer 43 has a rectangular shape.
The bottom electrode 45 is formed on the central portion and on the lateral borders of the supporting layer 43. The active layer 47 is formed on the bottom electrode 45 and the top electrode 49 is formed on the active layer 47. The bottom electrode 45 has a U-shape. The active layer 47 is smaller than the bottom electrode 45 and has the same shape as that of the bottom electrode 45. The top electrode 49 is smaller than the active layer 47 and has the same shape as that of the active layer 47. When a first signal is applied to the bottom electrode 45 and a second signal is applied to the top electrode 49, an electric field is generated between the top electrode 49 and the bottom electrode 45, so the active layer 47 is deformed by the electric field.
The via contact 53 is formed in a via hole 51 which is formed from a portion of the active layer 47 to the connecting terminal 33 through the bottom electrode 45, the supporting layer 43, the etching stop layer 37 and the passivation layer 35. The via contact 53 connects the bottom electrode 45 to the connecting terminal 33.
The reflecting member 55 for reflecting an incident light is formed at the central portion of the supporting layer 43. The reflecting member 55 has a predetermined thickness from the surface of the supporting layer 43 to a side of the active layer 47. Preferably, the reflecting member 55 is a mirror which has a rectangular shape.
A method for manufacturing the thin film AMA will be described as follows.
FIGS. 5A to 5D illustrate manufacturing steps of the thin film AMA.
Referring to FIG. 5A, the passivation layer 35 is formed on the substrate 31 having the electrical wiring (not shown) and the connecting terminal 33. The electrical wiring and the connecting terminal 33 receive the first signal (picture signal) from outside and transmit the first signal to the bottom electrode 45. Preferably, the electrical wiring has a metal oxide semiconductor (MOS) transistor for switching operation. The connecting terminal 33 is formed by using a metal, for example tungsten (W). The connecting terminal 33 is connected to the electrical wiring. The passivation layer 35 is formed by using phosphor-silicate glass (PSG) and by chemical vapor deposition (CVD) method so that the passivation layer 35 has a thickness of between 0.1 .mu.m and 1.0 .mu.m. The passivation layer 35 protects the substrate 31 having the electrical wiring and the connecting terminal 33 during subsequent manufacturing steps.
The etching stop layer 37 is formed on the passivation layer 35 by using a nitride and by low pressure chemical vapor deposition (LPCVD) method so that the etching stop layer 37 has a thickness of between 1000 .ANG. and 2000 .ANG.. The etching stop layer 37 protects the passivation layer 35 and the substrate 31 during subsequent etching steps.
A sacrificial layer 39 is formed on the etching stop layer 37 by using a PSG and by atmospheric pressure CVD (APCVD) method so that the sacrificial layer 39 has a thickness of between 0.5 .mu.m and 4.0 .mu.m. In this case, the degree of flatness of the sacrificial layer 39 is poor because the sacrificial layer 39 covers the top of the substrate 31 including the electrical wiring and the connecting terminal 33. Therefore, the surface of the sacrificial layer 39 is planarized by using a spin on glass (SOG) or by chemical mechanical polishing (CMP) method. Subsequently, a first portion of the sacrificial layer 39 having the connecting terminal 33 thereunder and a second portion of the sacrificial layer 39 which is adjacent to the first portion of the sacrificial layer 39 are etched in order to expose a first portion of the etching stop layer 37 having the connecting terminal 33 thereunder and a second portion of the etching stop layer 37 which is adjacent to the first portion of the etching layer 37 with respect to form the supporting layer 43.
Referring to FIG. 5B, a first layer is formed on the first and second portions of the etching stop layer 37 and on the sacrificial layer 39. The first layer is formed by using a rigid material such as a nitride or a metal. The first layer is formed by LPCVD method so that the first layer has a thickness of between 0.1 .mu.m and 1.0 .mu.m. The first layer will be patterned so as to form the supporting layer 43.
A bottom electrode layer is formed on the first layer by using an electrically conductive metal such as platinum (Pt), tantalum (Ta) or platinum-tantalum (Pt--Ta). The bottom electrode layer is formed by sputtering method or CVD method so that the bottom electrode layer has a thickness of between 0.1 .mu.m and 1.0 .mu.m. Subsequently, the bottom electrode layer is iso-cutted in order to separate each bottom electrode layer, so each pixel of the thin film AMA independently receives the first signal from outside through the electrical wiring and the connecting terminal 33. The bottom electrode layer will be patterned to form the bottom electrode 45.
A second layer is formed on the bottom electrode layer by using a piezoelectric material such as PZT (Pb(Zr, Ti)O.sub.3) or PLZT ((Pb, La)(Zr, Ti)O.sub.3) so that the second layer has a thickness of between 0.1 .mu.m and 1.0 .mu.m. Preferably, the second layer has a thickness of about 0.4 .mu.m. After the second layer is formed by sol-gel method, sputtering method or CVD method, the second layer is annealed by rapid thermal annealing (RTA) method. The second layer will be patterned to form the active layer 47.
A top electrode layer is formed on the second layer by using an electrically conductive metal, for example, aluminum (Al), platinum or silver (Ag). The top electrode layer is formed by sputtering method or CVD method so that the top electrode layer has a thickness of between 0.1 .mu.m and 1.0 .mu.m. The top electrode layer also will be patterned to form the top electrode 49.
Referring to FIG. 5C, after a first photo resist (not shown) is coated on the top electrode layer by spin coating method, the top electrode layer is patterned to form the top electrode 49 by using the first photo resist as an etching mask. As a result, the top electrode 49 has a U-shape. The second signal (bias signal) is applied to the top electrode 49 for generating the electric field between the top electrode 49 and the bottom electrode 45.
A second photo resist (not shown) is coated on the top electrode 49 and on the second layer by spin coating method after the first photo resist is removed by etching. The second layer is patterned to form the active layer 47 by using the second photo resist as an etching mask. The active layer 47 has a U-shape which is wider than that of the top electrode 49. After the second photo resist is removed by etching, a third photo resist (not shown) is coated on the top electrode 49, on the active layer 47 and on the bottom electrode layer by spin coating method. The bottom electrode layer is patterned to form the bottom electrode 45 by using the third photo resist as an etching mask. The bottom electrode 45 has a U-shape which is wider than that of the active layer 47. Then, the third photo resist is removed by etching.
Subsequently, portions of the active layer 47, the bottom electrode 45, the first layer, the etching stop layer 37 and the passivation layer 35 are etched so as to form the via hole 51 from the portion of the active layer 47 to the connecting terminal 33. The via contact 53 is formed in the via hole 51 by using an electrically conductive metal such as tungsten (W), platinum, aluminum or titanium. The via contact 53 is formed by sputtering method or CVD method so that the via contact 53 is formed from the connecting terminal 33 to the bottom electrode 45. The via contact 53 connects the bottom electrode 45 to the connecting terminal 33.
Referring to FIG. 5D, the first layer is patterned to form the supporting layer 43 by using a fourth photo resist (not shown) as an etching mask after the fourth photo resist is coated on the bottom electrode 45 by spin coating method. The supporting layer 43 has the lateral borders and the central portion. The bottoms of the lateral borders of the supporting layer 43 are partially attached to the etching stop layer 37 and are called anchors 43a, 43b. The lateral borders of the supporting layer 43 are formed parallel to and above the etching stop layer 37 from the attached portions. The central portion of the supporting layer 43 is integrally formed with the lateral borders between the lateral borders. The central portion of the supporting layer 43 has a rectangular shape. Then, the fourth photo resist is removed by etching. A portion of sacrificial layer 39 is exposed when the first layer is patterned.
After a fifth photo resist (not shown) is coated on the exposed portion of the sacrificial layer 39 and on the supporting layer 43 by spin coating method, the fifth photo resist is patterned to expose the central portion of the supporting layer 43. The reflecting member 55 is formed on the central portion of the supporting layer 43 by using a reflective material such as silver, platinum, or aluminum. The reflecting member 55 is formed by sputtering method or CVD method so that the reflecting member 55 has a thickness of between 0.3 .mu.m and 2.0 .mu.m. The reflecting member 55 for reflecting the incident light from a light source (not shown) has the same shape as that of the central portion of the supporting layer 43. Subsequently, the fifth photo resist and the sacrificial layer 39 are removed by using hydrogen fluoride (HF) vapor, so the actuator 57 is complete. When the sacrificial layer 39 is removed, an air gap 41 is formed where the sacrificial layer 39 is located.
The first signal is applied to the bottom electrode 45 from outside through the electrical wiring, the connecting terminal 33 and the via contact 53. At the same time, when the second signal is applied to the top electrode 49 from a common line (not shown), the electric field is generated between the top electrode 49 and the bottom electrode 45. The active layer 47 formed between the top electrode 49 and the bottom electrode 45 is deformed by the electric field. The active layer 47 is deformed in the direction perpendicular to the electric field. The actuator 57 having the active layer 47 is actuated in the opponent direction to the position where the supporting layer 43 is positioned. That is, the actuator 57 is actuated upward and the supporting layer 43 attached to bottom electrode 45 is also actuated upward according to the tilting of the actuator 57.
The reflecting member 55 reflecting the incident light from the light source is tilted with the actuator 57 because the reflecting member 55 is formed at the central portion of the supporting layer 43. Hence, the reflecting member 55 reflects the light onto a screen so the picture is formed on the screen.
However, in the above-described thin film AMA, cracks are generated from the portion of the second layer (the active layer) formed at iso-cutted portion of the bottom electrode layer into the other portion of the second layer because the second layer is formed on the bottom electrode layer after the bottom electrode layer is iso-cutted in order to separate the pixels of the thin film AMA. Therefore, an electrical short may be occurred between the top electrode and the bottom electrode since the top electrode and the bottom electrode are partially connected each other through the cracks generated in the active layer. When the electrical short is generated, the actuator cannot be actuated so the point defect of pixel may be occurred in the thin film AMA.
Also, the actuator may be initially tilted without applying of the first and the second signals because the deformation driving force such as uneven residual stresses and stress gradient are applied to the stress concentration line which is generated when the sacrificial layer is patterned so as to form the anchors for supporting the actuator. Therefore, the light efficiency of the incident light may be decrease because the reflecting member does not have a desired tilting angle when the actuator is initially tilted, so the quality of the picture projected onto the screen is decreased.